The laser gyro, developed some 30 years ago, is widely used on a commercial scale at the present time. Its principle of operation is based on the Sagnac effect, which induces a frequency difference Δν between the two optical transmission modes that propagate in opposite directions, called counterpropagating modes, of a bidirectional laser ring cavity undergoing a rotational motion. Conventionally, the frequency difference Δν between the two optical modes induced by the rotational motion is equal to:Δν=4AΩ/λL where L and A are the length and the area of the cavity, respectively; λ is the laser emission wavelength excluding the Sagnac effect; and Ω is the rotation speed of the assembly. By measuring Δν, obtained by spectral analysis of the beat between the two emitted beams, it is possible to determine the value of Ω very accurately. A typical fringe counting device for laser gyros is used, based on the beat signal, to determine the relative angular position of the system.
In standard laser gyros, the amplifying medium is a gaseous medium of helium and neon atoms in appropriate proportions. However, the gaseous nature of the amplifying medium is a source of technical complications when producing the laser gyro, notably because of the high gas purity required and the premature wear of the cavity during its use due, in particular, to gas leakage and to deterioration of the high-voltage electrodes used to establish the population inversion.
It is possible to produce a solid-state laser gyro operating in the visible or near infrared using, for example, an amplifying medium based on crystals doped with ions of the rare earth type, such as neodymium, erbium or ytterbium, instead of helium/neon gas mixtures, the optical pumping then being provided by lasers diode operating in the near infrared. Thus, all the problems inherent with the gaseous state of the amplifying medium are de facto eliminated.
However, this type of laser gyro construction has certain technical difficulties partly due to the fact that the counterpropagating waves interfere within the amplifying medium.
This is because, if the amplifying medium is a crystalline solid of the Nd:YAG type, it can be demonstrated that, in such a medium, the population inversion gratings induced by stimulated emission in the gain medium have the effect of destabilizing the bidirectional emission. In addition, when the laser gyro is rotating, these gratings become moving gratings and induce, by the Doppler effect, a frequency shift between the two counterpropagating waves circulating in the laser cavity, thereby increasing the nonlinearity of the frequency response of the laser gyro.
It is also possible to use as amplifying medium a semiconductor with a vertical structure of the VECSEL (Vertical External Cavity Surface Emitting Laser) type. A VECSEL essentially comprises a stack of active quantum well zones constituting gain zones. For gyroscope applications, the use of a vertical structure is advantageous in so far as the gain zones may have a diameter of around 100 microns, close to the dimensions of the optical beam circulating in the cavity, also allowing propagation of the unguided wave. However, the use of such a device in transmission is excluded. This is because the active quantum well zones of the vertical structure must have a pitch equal to that of the grating formed by the interference between the two counterpropagating waves present in this structure so as to optimize the gain. When the laser gyro is rotating, the optical grating is not free to move as its maxima, also called antinodes, must remain within the gain zones. In this case, “gain-induced frequency locking” is obtained, which in fact makes the device unusable as a laser gyro.